Monoclonal antibodies or other ligands are potentially useful for the diagnosis and treatment of tumors in lymph nodes. Their therapeutic and diagnostic potential depends on the ability of the antibody to reach the target cell. We have developed a theoretical model for the transport of monoclonal antibodies and water into the lymphatic and capillary systems following subcutaneous injection. The model incorporates processes for transcapillary and translymphatic solvent and solute movement that account for a) hydrostatic and osmotic pressure differences between the injected solution and fluid surrounding the injection site, b) differences in the available pore area for transport into the lymphatic and capillary systems and c) specific and nonspecific binding of antibody molecules to tissue cells at the injection site. The partial differential equations describing the model are being solved numerically on a VAX/11-780 computer. Significant theoretical findings to date include the following: 1) most of the antibody that leaves the injection site to enter the lymphatics does so by convection in the fluid also entering the lymphatics, 2) most of the water leaving the injection site does so entering the capillary system 3) the repeated administration of smaller doses of antibody over longer times would improve delivery into the lymphatic system and 4) the inclusion of an osmotic agent in the injection solution would tend to reduce water loss into the capillary system and improve antibody entry into the lymphatic system. The concepts arising from this study are directly applicable to the design of clinical studies with monoclonal antibodies and other ligands.